In many areas of technology (e.g., optical data processing, optical communications) it is necessary to modulate optical radiation [including continuous wave ("CW") radiation] in accordance with a given signal. A conventional technique for modulating the output of a semiconductor laser is the modulation of the laser drive (pump) current. This technique can be used for modulation frequencies up to the so-called electron-photon resonance (EPR) frequency, which typically is of order 10 GHz. However, for fundamental reasons the above conventional technique does not result in efficient modulation of the laser output for modulation frequencies substantially above the EPR frequency. However, it can be expected that the trend towards higher and higher bit rates in, e.g., optical communications systems will continue, demanding means for modulating optical radiation at higher and higher frequencies, ultimately at frequencies above the EPR frequency.
Recent work in the USSR inter alia has shown that the optical output of a semiconductor heterojunction laser can be modulated at frequencies above the EPR frequencies, exemplarily as high as 100 GHz or even higher, by a novel mechanism that involves heating of the free electrons in the active layer of the laser by an electric field applied parallel to the active layer, driving a lateral electric current through that layer. See V. B. Gorfinkel et al., Soviet Physics Semiconductor, Vol. 24(4), April 1990, p. 466; S. A. Gurevich et al., Joint Soviet-American Workshop on the Physics of Semiconductor Lasers, May 20-Jun. 3, 1991, p. 67; and V. B. Gorfinkel et al., International Journal of Infrared and Millimeter Waves, June 1991, all incorporated herein by reference. In the cited publications is disclosed a AlGaAs-GaAs separate-confinement heterostructure single quantum well (QW) ridge laser that comprises, in addition to the conventional pumping contact, two contacts for carrier heating. By means of current pulses applied to the heating contacts it was possible to raise the temperature of the free carriers about 100K above the ambient (lattice) temperature, resulting in modulation of the laser output.
FIG. 1 schematically illustrates modulation in the above discussed laser. Curve 11 represents the conventional situation, namely, laser emission when the free carrier temperature is equal to the lattice temperature. Raising the free carrier temperature above the lattice temperature by means of the heating current results in increased loss in the active region, shifting the lasing onset to a higher drive current, resulting in curve 12. As can readily be seen, modulation of the free carrier temperature thus results in modulation of the laser power. For instance, if a constant drive current of value 13 is applied, the laser output power can be switched between 0 and value 14. Since the free carrier temperature can be raised very quickly and also relaxes to the lattice temperature extremely quickly, in a time of order 10.sup.-12 s, high modulation frequencies are in principle possible.
However, free carrier heating by means of electric current frequently will be undesirable, since the heating power derives solely from the logic controlling circuit. Moreover, in the prior art device relatively large power is dissipated by the holes, due to their large effective density of states. Furthermore, in practice it will frequently be difficult to achieve uniform heating of the free carriers, again resulting in performance deterioration. Thus, in view of the importance of efficient very high frequency modulation means, it would be highly desirable to have available such means that are substantially free of at least some of the above such discussed drawbacks. This application inter alia discloses such modulation means.